1. Field of the Invention
The present invention refers to a low noise amplifier, particularly to a radio frequency low noise amplifier.
2. Description of Related Art
In the field of telecommunication integrated devices a low noise amplifier is typically utilized as input stage for a receiver. Such amplifiers must assure besides a low noise in working frequency band thereof, even an efficient reduction of the distortion of the signals that are to be amplified.
Many problems of the same circuit of the amplifier are linked both to the optimisation of the noise and distortion performances and the control of the input impedance of the amplifier. Such problems have, above all in the radio frequency field, a basilar importance because the input impedance of the amplifier fixes, with a given source resistor, the adaptation conditions of the amplifier circuit at its interface with the input signal, thereby causing the ROS (Stationary Wave Ratio) of the receiver.
Open loop or feedback technologies can be utilized to control the input impedance of the low noise amplifiers. A typical circuit typology of a low noise amplifier that uses a feedback technology is shown in FIG. 1 where a differential stage is described which is constituted by two bipolar transistors Q10 and Q20 which have the emitter terminals connected with each other and with a current generator Iee, connected in turn with the voltage Vee, and the respective collector terminals connected with each emitter terminal of two bipolar transistors Q30 and Q40. Such transistors have each base terminal connected with a bias voltage VB and each collector terminal connected both with a resistor Rc, connected in turn with a supply voltage Vcc, and with a base terminal of the transistors Q50 and Q60 which have in turn the collector terminals connected with the supply voltage Vcc and the emitter terminals connected with current generators connected with the voltage Vee. The base terminals of the transistors Q10 and Q20, which represent the input terminals of the amplifier (which has in input a voltage VIN), are coupled with the emitter terminals of the transistors Q50 and Q60, which represent the output terminals OUTxe2x88x92 and OUT+ of the amplifier, by means of a resistor RF. In such amplifier the input impedance is fixed by the feedback resistor RF and it is equal to:       Ri    ≈                  2        ⁢        RF            Av        =            2      ⁢      RF                      g                  m1          ,          2                    ⁢      Rc      
wherein Av is the voltage gain of the amplifier circuit. Since the input impedance in such circuit configuration of the amplifier is fixed by a physical resistor and by a voltage gain, it results constant in a very large frequency band and therefore it makes this circuit configuration adapted to the large band appliances.
However a circuit configuration as that in FIG. 1 does not allow to solve the distortion problem. An added specification required to a low noise amplifier is therefore the gain variability, in fact an adjustable gain allows to optimise the distortion performances of the amplifier. The gain variation can be controlled statically by a regulation with an external trimming or a software programming, or it can be controlled by means of a closed loop technology with an AGC (Automatic Gain Control) circuit. An amplifier employing a technology of the above mentioned type is shown in FIG. 2 where a differential stage is shown which is constituted by two bipolar transistors Q11 and Q21 the emitter terminals of which are connected with each other and with a current generator Iee, connected in turn with the voltage Vee, and the respective collector terminals connected with other two differential stages Q31, Q41 and Q51, Q61 and specifically with their emitter terminals. The collector terminals of the transistors Q31 and Q51 are connected with a resistor Rc, connected in turn with a supply voltage Vcc, and with a base terminal of the transistor Q70, while the collector terminals of the transistors Q41 and Q61 are connected with a resistor Rc, connected in turn with the supply voltage Vcc, and with the base terminal of the transistor Q80. The collector terminals of the transistors Q70 and Q80 are connected with the supply voltage Vcc while each one of the emitter terminals, which constitute respectively the output terminals OUTxe2x88x92 and OUT+, is connected with a current generator connected with the voltage Vee. The base terminals of the transistors Q11 and Q21 constitute the input terminals of the amplifier (which has in input a voltage VIN), while between the common base terminal of the transistors Q41 and Q51 and the common base terminal of the transistors Q31 and Q61 a voltage Vc is applied which serves to regulate the gain of the low noise amplifier. By such a circuit configuration a maximum gain is obtained equal to:
AvMAX=gm1,2Rc
for example with a voltage Vcxe2x89xa70.2V, while the minimum gain is obtained for example with Vc=0 and it is limited only by the incorrect coupling (mismatch) of the transistor couples belonging to the three differential stages.
However in such circuit configuration the input impedance cannot be correctly controlled; in fact the insertion of the variable gain does not allows to achieve a constant input impedance.
In view of the art described, it is an object of the present invention to provide a low noise amplifier that overcomes the aforementioned problem.
According to the present invention, such object is obtained by means of a low noise amplifier comprising a first circuit block suitable for converting a first amplifier input voltage signal into current, a second circuit block adapted to divide the current coming from said first block, said second block being controlled by a second voltage signal, said first and second blocks conferring a variable voltage gain to the amplifier, characterised by comprising at least one first and at least one second resistors and a feedback network said at least one first resistor connected with one first output terminal of said second block and with a supply voltage, and said at least one second resistor being connected between said at least one first and at least one second output terminals of said second block, and said feedback network being coupled with said at least one first terminal and with said first circuit block, and said at least one second terminal being coupled with at least one output terminal of said low noise amplifier.
Thanks to the present invention it is possible to provide a low noise amplifier having the variable gain and fixed input impedance features.